/Engine/Source/Runtime/CoreUObject/Public/Misc/PackagePath.h
Note this is not the same asFPaths::GetExtensionbecause some EPackageExtensions have multiple '.'s (e.g. .m.ubulk)
The EPackageExtension matching the (case-insensitive) extension text in the filename. EPackageExtension::Unspecified if the filename has no extension. EPackageExtension::Custom if the filename's extension is not one of the enumerated possibilities in EPackageExtension
static EPackageExtension ParseExtension ( FStringView Filename, int32 * OutExtensionStart )